The physical elements in a packet-based data communication system commonly include multi-port units such as switches and routers. Such units are commonly and conveniently manufactured with a fixed plurality of ports by means of which the units can be connected by way of appropriate transmission links (such as cables or optical fibres) to other units or user terminals. In order to provide greater versatility in constructing networks while minimising the number of different sizes of units that need to be deployed it is known as described in EP-0912944 and also various switches such as the Super Stack 3 Switch 3300 made by 3Com Corporation, to render the units ‘stackable’ by which is meant that a multiplicity of them can be interconnected to form effectively a single controllable entity. It is customary to make such units physically stackable in a column although this physical aspect of the term is not essential to the meaning of ‘stackable’ or to the present invention.
In order to provide intercommunication between the units, so that for example packets received at any of the ordinary ports (commonly termed ‘front panel’ ports) can be forwarded from a port or ports on another unit or units, (depending on the addressing of the packets and the nature of them, it is customary to connect stacked units together by means of a cascade, which is the term used for the communication links and associated control functions by means of which packets are transmitted between the units in the stack.
The design and organisation of a cascade are attended by some considerable complexity. For example, the forwarding of packets on the cascade has to be controlled according to a variety of rules which are intended to conform, for each particular unit, to the forwarding rules relevant for different types of packet, such as unicast, broadcast and multicast, as well as other relevant rules such as those relating to bridging. The rules may need to be preserved or modified to make the operation of the cascade more convenient or more versatile. Furthermore, as will be more particularly appreciated hereinafter, it is desirable to employ a packet format which accommodates information particular to the operation of the cascade.
Although very simple cascades can be constituted by means of ordinary communication links between ports on a stack of similar or compatible devices, a variety of operational desiderata or network constraints require a cascade connection to be rather more sophisticated and produce greater difficulties in design. For example, if a high performance cascade connection is to be maintained while providing fair access to the cascade connection for all the units in the stack, it is a practical desirability for the conveyance of certain control or status information to the all the units of the stack so that, for example, an arbitration protocol may be observed. Such an arbitration protocol usually requires such data as the number of units in the stack and preferably also some identification number for each of those units. Furthermore, it is a desirable feature of a cascaded stack that notwithstanding the powering-down or failure of one of the units in the stack, it can continue to operate in respect of the remaining units in the stack. A similar desirable feature is the accommodation of ‘hot’ insertion or swapping of units in the stack.
To some extent information in respect of the stack may be accommodated within packets which are forwarded onto the cascade. Examples are given by published GB patent applications 2359692 and GB-2361830 which allow for the incorporation within a packet header of fields that identify which of the units have ports that are members of a trunked connection to the stack. However, the inclusion of control or status data relating to the units within packet headers is generally inconvenient if at to all feasible.
It is know from the 3Com switch type 4400 to provide a cascaded stack of units for which a data path (for packets) and a control path (for status and control information) can be maintained despite the powering-down of one or more units. The system relies on 3-port connectors which have a port for connection of data packets and control frames to and from an associated unit and two other ports which maintain a data path and a control path around the cascade. The system is quite versatile but the connectors, known as T-pieces, employed in the practical system represent a considerable hardware ‘overhead’ which is particularly significant as the operating frequencies increase, owing to, for example, the need for active clock recovery circuits.